Each reference from the following list of references is incorporated herein by reference:    1. A. C. Fischer-Cripps, Nanoindentation (Springer, New York, 2004). (“Reference 1”)    2. “Review of instrumented indentation”, M. R. VanLandingham, J. Res. Natl. Inst. Stand. Technol. 108, 249 (2003). (“Reference 2”)    3. “An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation measurements”, W. C. Oliver and G. M. Pharr, J. Mater. Res. 7, 1564 (1992). (“Reference 3”)    4. “Challenges and interesting observations associated with feedback-controlled nanoindentation”, O. L. Warren, S. A. Downs, and T. J. Wyrobek, Z. Metallkd. 95, 287 (2004). (“Reference 4”)    5. For a recent review of in-situ TEM techniques: “New developments in transmission electron microscopy for nanotechnology”, Z. L. Wang, Adv. Mater. 15, 1497 (2003). (“Reference 5”)    6. “Development of an in situ nanoindentation specimen holder for the high voltage electron microscope”, M. A. Wall and U. Dahmen, Microsc. Microanal. 3, 593 (1997). (“Reference 6”)    7. “An in situ nanoindentation specimen holder for a high voltage transmission electron microscope”, M. A. Wall and U. Dahmen, Microsc. Res. Tech. 42, 248 (1998). (“Reference 7”)    8. “Development of a nanoindenter for in situ transmission electron microscopy”, E. A. Stach, T. Freeman, A. M. Minor, D. K. Owen, J. Cumings, M. A. Wall, T. Chraska, R. Hull, J. W. Morris, Jr., A. Zettl, and U. Dahmen, Microsc. Microanal. 7, 507 (2001). (“Reference 8”)    9. “Quantitative in situ nanoindentation in an electron microscope”, A. M. Minor, J. W. Morris, and E. A. Stach, Appl. Phys. Lett. 79, 1625 (2001). (“Reference 9”)    10. “In-situ transmission electron microscopy study of the nanoindentation behavior of Al”, A. M. Minor, E. T. Lilleodden, E. A. Stach, and J. W. Morris, Jr., J. Electr. Mater. 31, 958 (2002). (“Reference 10”)    11. “In-situ nanoindentation—a unique probe of deformation response in materials”, A. Minor, E. Lilleodden, M. Jin, E. Stach, D. Chrzan, B. Morris, T. Friedmann, X. Xiao, J. Carlisle, and O. Auciello, Microsc. Microanal. 9, 900 (2003). (“Reference 11”)    12. “An in-situ TEM nanoindenter system with 3-axis inertial positioner”, M. S. Bobji, C. S. Ramanujan, R. C. Doole, J. B. Pethica, and B. J. Inkson, Mater. Res. Soc. Symp. Proc. 778, U4.5.1 (2003). (“Reference 12”)    13. “Direct observations of incipient plasticity during nanoindentation of Al”, A. M. Minor, E. T. Lilleodden, E. A. Stach, and J. W. Morris, Jr., J. Mater. Res. 19, 178 (2004). (“Reference 13”)    14. “Dislocation-grain boundary interactions in martensitic steel observed through in situ nanoindentation in a transmission electron microscope”, T. Ohmura, A. M. Minor, E. A. Stach, and J. W. Morris, Jr., J. Mater. Res. 19, 3626 (2004). (“Reference 14”)    15. “Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al—Mg thin films”, W. A. Soer, J. Th. M. De Hosson, A. M. Minor, J. W. Morris, Jr., and E. A. Stach, Acta Mater. 52, 5783 (2004). (“Reference 15”)    16. “Study of deformation behavior of ultrafine-grained materials through in situ nanoindentation in a transmission electron microscope”, M. Jin, A. M. Minor, D. Ge, and J. W. Morris, Jr., J. Mater. Res. 20, 1735 (2005). (“Reference 16”)    17. “Room temperature dislocation plasticity in silicon”, A. M. Minor, E. T. Lilleodden, M. Jin, E. A. Stach, D. C. Chrzan, and J. W. Morris, Jr., Philos. Mag. 85, 323 (2005). (“Reference 17”)    18. “Indentation mechanics of Cu—Be quantified by an in situ transmission electron microscopy mechanical probe”, M. S. Bobji, J. B. Pethica, and B. J. Inkson, J. Mater. Res. 20, 2726 (2005). (“Reference 18”)    19. Brochure from Gatan Inc. titled “In situ probing: STM-TEM systems from Nanofactory™ Instruments”. (“Reference 19”)    20. Presentation from Gatan Inc. titled “TEM-nanoindentor system from Nanofactory”, authored by Oleg Lourie, Gatan Inc. (“Reference 20”)    21. ISO 14577-1:2002 (“Metallic materials—instrumented indentation test for hardness and materials parameters—part 1: test method”); ISO 14577-2:2002 (“Metallic materials—instrumented indentation test for hardness and materials parameters—part 2: verification and calibration of testing machines”); ISO 14577-3:2002 (“Metallic materials—instrumented indentation test for hardness and materials parameters—part 3: calibration of reference blocks”). (“Reference 21”)    22. “Effect of PZT and PMN actuator hysteresis and creep on nanoindentation measurements using force microscopy”, S. M. Hues, C. F. Draper, K. P. Lee, and R. J. Colton, Rev. Sci. Instrum. 65, 1561 (1994). (“Reference 22”)    23. “The effect of instrumental uncertainties on AFM indentation measurements”, M. R. VanLandingham, Microsc. Today 97, 12 (1997). (“Reference 23”)    24. “Quantification issues in the identification of nanoscale regions of homopolymers using modulus measurement via AFM nanoindentation”, C. A. Clifford and M. P. Seah, Appl. Surf. Sci. 252, 1915 (2005). (“Reference 24”)    25. “A micromachined nanoindentation force sensor”, A. Nafari, A. Danilov, H. Rödjegård, P. Enoksson, and H. Olin, Sens. Actuators, A, 123-124, 44 (2005). (“Reference 25”)    26. Brochure from Hysitron, Inc. titled “TriboIndenter®: nanomechanical test instruments”; brochure from Hysitron, Inc. titled “Ubi 1®: scanning quasistatic nanoindentation”; brochure from Hysitron, Inc. titled “TriboScope®: quantitative nanomechanical testing for AFMs”. (“Reference 26”)    27. “A new force sensor incorporating force-feedback control for interfacial force microscopy”, S. A. Joyce and J. E. Houston, Rev. Sci. Instrum. 62, 710 (1991). (“Reference 27”)    28. “Interfacial force microscopy: a novel scanning probe technique for imaging and quantitative measurement of interfacial forces and nanomechanical properties”, O. L. Warren, J. F. Graham, and P. R. Norton, Phys. Can. 54, 122 (1998). (“Reference 28”)    29. “Apparatus for microindentation hardness testing and surface imaging incorporating a multi-plate capacitor system”, W. A. Bonin, U.S. Pat. Nos. 5,553,486 and 6,026,677. (“Reference 29”)    30. “Capacitive transducer with electrostatic actuation”, W. A. Bonin, U.S. Pat. No. 5,576,483. (“Reference 30”)    31. “Multi-dimensional capacitive transducer”, W. A. Bonin, U.S. Pat. Nos. 5,661,235 and 5,869,751. (“Reference 31”)    32. “Nanoindentation and picoindentation measurements using a capacitive transducer system in atomic force microscopy”, B. Bhushan, A. V. Kulkarni, W. Bonin, and J. T. Wyrobek, Philos. Mag. A 74, 1117 (1996). (“Reference 32”)    33. For example: “Vertical comb-finger capacitive actuation and sensing for CMOS-MEMS”, H. Xie and G. K. Fedder, Sens. Actuators, A 95, 212 (2002). (“Reference 33”)    34. “Force-sensing system, including a magnetically mounted rocking element”, J. E. Griffith and G. L. Miller, U.S. Pat. No. 5,307,693. (“Reference 34”)    35. “A rocking beam electrostatic balance for the measurement of small forces”, G. L. Miller, J. E. Griffith, E. R. Wagner, and D. A. Grigg, Rev. Sci. Instrum. 62, 705 (1991). (“Reference 35”)    36. “High-resolution capacitive load-displacement transducer and its application in nanoindentation and adhesion measurements”, N. Yu, W. A. Bonin, and A. A. Polycarpou, Rev. Sci. Instrum. 76, 045109 (2005). (“Reference 36”)    37. “Tapping mode imaging with an interfacial force microscope”, O. L. Warren, J. F. Graham, and P. R. Norton, Rev. Sci. Instrum. 68, 4124 (1997). (“Reference 37”)    38. “Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer”, S. A. S. Asif, K. J. Wahl, and R. J. Colton, Rev. Sci. Instrum. 70, 2408 (1999). (“Reference 38”)    39. “Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation”, S. A. S. Asif, K. J. Wahl, R. J. Colton, and O. L. Warren, J. Appl. Phys. 90, 1192 (2001); Erratum 90, 5838 (2001). (“Reference 39”)    40. “High-performance drive circuitry for capacitive transducers”, W. Bonin, U.S. Pat. No. 6,960,945. (“Reference 40”)
Nanoindentation (see References 1 and 2), today's primary technique for probing small volumes of solids for the purpose of quantifying their mechanical properties, involves the use of an instrument referred to as a nanoindenter to conduct a nanoindentation test. At a minimum, a nanoindentation test entails a gradual loading followed by a gradual unloading of a sharp indenter against a sample. The indenter is usually made of diamond, diamond being both the stiffest and the hardest known material. The indenter is shaped to a well-defined geometry typically having an apical radius of curvature in the range of 50-100 nm. The most prevalent indenter geometry is the three-sided pyramidal Berkovich geometry, which imposes a representative strain of ˜7% if perfectly formed.
A hallmark of nanoindentation is the acquisition throughout the nanoindentation test of both the force applied to the sample (peak load typically <10 mN) and the indenter displacement into the sample (maximum penetration depth typically <10 μm) to generate a force-displacement curve. High-performance nanoindenters exhibit force and displacement noise floors below 1 μN RMS and 1 nm RMS, respectively. The sample's mechanical properties, such as elastic modulus and hardness, can be evaluated by analyzing the force-displacement curve, the most common method of analysis being the elastic unloading analysis published by Oliver and Pharr (see Reference 3) in 1992.
Nanoindentation suffers from a major shortcoming, however. Despite more than a decade's worth of maturation, nanoindentation still leaves much to be desired in terms of providing definitive mechanistic explanations for certain features of its outputted force-displacement curves. For example, the commonly observed load-controlled nanoindentation phenomenon of a pop-in transient (see Reference 4), a sudden sizeable increase in penetration depth without a corresponding increase in load, an event signaling discontinuous yielding, has many possible interpretations: dislocation burst, shear band formation, fracture onset, spalling, stress-induced phase transformation, etc. Because it is extremely difficult to image such discrete nano-to-atomistic scale happenings at their moments of occurrence, it is not surprising that the scientific literature is replete with examples of deformation mechanisms assigned to pop-in transients with little more to go on than knowledge of the nature of the sample under investigation in combination with educated speculation. The invention provides the opportunity to make unambiguous the microscopic origin of a pop-in transient, or that of any other encountered nanoindentation phenomenon, by coupling nanoindentation to a TEM in an in-situ manner (see Reference 5). Doing so required meeting a set of configurational and environmental challenges not anticipated by existing nanoindentation transducers.
Configurational challenges presented by TEMs include: (1) severely restricted space mandating a nanoindentation transducer considerably more miniature than those currently supplied with commercial nanoindenters; (2) achieving acceptably high maximums in load and penetration depth in spite of the limited size of the transducer; (3) the need to operate the transducer with its indenter horizontal rather than in the standard vertical orientation; (4) the requirement that the indenter extend significantly from the transducer's body to reach well into the TEM's pole piece gap, which necessitates means for countering the associated tilting moment; (5) the requirement that the transducer be largely insensitive with respect to being rotated about the indenter's axis; and (6) the requirement that the transducer achieve high performance in spite of long wiring runs from the transducer residing in vacuum to its electronic circuitry residing out of vacuum, the longer the wiring runs, the greater the likelihood of electromagnetic interference pick-up and capacitive signal loading.
Environmental challenges presented by TEMs include: (1) high vacuum (e.g., 10−7 torr) limiting construction materials to those not prone to outgassing; (2) the requirement that the transducer not seriously impede the pumping conductance of the TEM holder so that high vacuum can be achieved in a sensible period of time; (3) high vacuum restricting actuation/sensing strategies to those generating minimal heat; (4) high vacuum increasing the transducer's mechanical quality factor (Q) to a value much higher than in air, the higher the quality factor, the longer the impulse-ring-down time; (5) the presence of a highly energetic electron beam (e.g., 300 kV) impinging the indenter, which necessitates means for bleeding charge from the indenter; and (6) the presence of an especially strong magnetic field (e.g., 2 tesla in magnitude) restricting actuation/sensing strategies to those not relying on magnetic principles, and limiting construction materials to those without ferromagnetic content.
Owing to the severe set of challenges to overcome, previous attempts at in-situ TEM nanoindentation (see References 6-20) have been limited to qualitative or semi-quantitative experimentation. Qualitative in-situ TEM nanoindentation refers to viewing/recording a stream of TEM images that show how a sample deforms during the nanoindentation process without having the technology to acquire a corresponding force-displacement curve. The inability to acquire a force-displacement curve renders this experimental approach of low relevance to the invention. Semi-quantitative in-situ TEM nanoindentation also refers to viewing/recording a stream of TEM images that show how a sample deforms during the nanoindentation process, but with the added dimension of acquiring a corresponding force-displacement curve of poor accuracy relative to metrological standards established for nanoindentation, such as those expressed in ISO 14577 (see Reference 21).
Further discussion of semi-quantitative nanoindentation helps to clarify the meaning of quantitative nanoindentation (quantitative nanoindentation is often referred to as depth-sensing indentation). The in-situ TEM nanoindenter manufactured by Nanofactory Instruments AB (TEM-Nanoindentor: SA2000.N (see References 19 and 20)) is a highly relevant example of a semi-quantitative nanoindenter. Nanofactory's instrument is at odds with metrological standards established for nanoindentation on account of the series loading configuration it adopts. The series loading configuration poses a problem because it does not provide a direct measure of penetration depth. Instead, ignoring factors such as load frame compliance and thermally-induced relative position drift, the penetration depth is equal to the motion provided by an actuator minus the deflection associated with a device inferring load. The change in deflection is virtually equal to the change in motion in the limit of high contact stiffness, where “high” means high relative to the spring constant of the deflectable device inferring load. Consequently, it is virtually impossible to resolve changes in penetration depth in the high contact stiffness limit, a limit very easily reached. In contrast, quantitative nanoindenters exhibit constant penetration depth resolution regardless of the value of the contact stiffness.
To further complicate matters, Nanofactory's instrument relies on a piezoelectric actuator to affect the indenter-sample separation, but the instrument does not have a displacement sensor dedicated to measuring the actuator's extension or contraction (see Reference 20). Computing a piezoelectric actuator's extension or contraction from the voltage applied to the actuator has been shown to be unreliable because such actuators exhibit non-linearity, hysteresis, and creep dependent on the history of use (see Reference 22). Sequential analysis of TEM images that show the indenter penetrating the sample seems to be a viable way of directly quantifying the penetration depth in the absence of direct depth sensing. However, our own experience tells us this method is inconvenient and of dubious accuracy. Moreover, the indenter cannot be seen in dark-field TEM images. Operationally, Nanofactory's instrument is reminiscent of an atomic force microscope (AFM) conducting nanoindentation. There is a long history of AFMs delivering faulty force-displacement curves partially on account of the difficulties just mentioned (see References 23 and 24).
In Nanofactory's instrument, the deflectable device inferring load is a miniature two-plate capacitive transducer (see Reference 25) comprising a stationary electrode and a spring-supported displaceable electrode to which the indenter is attached perpendicularly; “stationary” and “displaceable” mean stationary and displaceable with respect to the transducer's body. The displaceable electrode's deflection is determined by monitoring the change in capacitance. Multiplying the displaceable electrode's deflection by the spring constant of the springs supporting the displaceable electrode yields the force acting on the indenter. Curiously, Nanofactory's instrument does not capitalize its potential for electrostatic actuation (see Reference 20), which prevents it from employing a loading configuration other than the inappropriate series loading configuration.
A suite of nanoindenters manufactured by Hysitron, Inc. (see Reference 26) and the interfacial force microscope (IFM) (see References 27 and 28) originating from Sandia National Laboratories are scanning nanoindenters utilizing actuatable capacitive transducers. Both types of instruments are capable of raster scanning the indenter to image a sample's surface in the manner of an AFM. Useful information regarding deformation mechanisms can be obtained from post-test images of the indent's topography, but such images illustrate no more than the residual deformation field.
At the heart of Hysitron's nanoindenters is a patented three-plate capacitive transducer (see References 29-32) comprising two stationary electrodes and a spring-supported displaceable electrode to which the indenter is attached perpendicularly; “stationary” and “displaceable” mean the same as before. The electrodes are components of a three-plate stack, the displaceable electrode being an element of the center plate. Each stationary electrode has a center hole, one center hole passing through the indenter without hindrance and the other center hole with the purpose of equalizing electrode areas. The dual capability of electrostatic actuation and capacitive displacement sensing is a hallmark of Hysitron's three-plate capacitive transducer. Electrostatic actuation in this case refers to generating an electrostatic force between the displaceable electrode and the stationary electrode through which the indenter passes, which deflects the displaceable electrode with respect to the stationary electrodes. Capacitive displacement sensing in this case refers to sensing the deflection using the well-established differential capacitance half-bridge method involving all three electrodes now widely adopted by microelectromechanical systems (MEMS) (see Reference 33).
Hysitron's nanoindenters adopt a parallel loading configuration, meaning contact stiffness in parallel with the spring constant of the support springs. This loading configuration results in the transducer's capacitive displacement sensing output providing a direct measure of penetration depth, again ignoring factors such as load frame compliance and thermally-induced relative position drift. The calculation of contact force involves the applied electrostatic force and the spring force, the spring force being related to the product of the easily-calibrated spring constant of the support springs and the displaceable electrode's deflection.
At the heart of the IFM is a differential-capacitance displacement sensor (see Reference 27) (IFM sensor for brevity) comprised of two co-planar stationary electrodes facing a torsion-bar-supported rotatable electrode; “stationary” and “rotatable” mean stationary and rotatable with respect to the sensor's body. The rotatable electrode together with a pair of torsion bars extending from opposing edges of the rotatable electrode resembles a torsional pendulum. The indenter is attached perpendicularly to the outer face of the rotatable electrode at a position equivalent to one stationary electrode's center. A hallmark of the IFM is its operation as a torque balance. An electrostatic-force-feedback controller is used to servo the indenter-side electrostatic torque to continuously suppress the rotatable electrode from rotating under the influence of the indenter-sample torque; the non-indenter-side electrostatic torque is held constant by the controller. The well-established differential capacitance half-bridge method involving all three electrodes is used to sense the rotational displacement of the rotatable electrode. But the action of the controller continuously nulls the sensor's capacitive displacement sensing output. The rocking beam sensor (see References 34 and 35) originating from Bell Laboratories is similar to the IFM sensor, but is used for critical dimensional metrology rather than for nanoindentation.
IFMs use a piezoelectric actuator to affect the indenter-sample separation. The motion provided by the piezoelectric actuator in combination with the stiffening action of the electrostatic-force-feedback controller permits direct control of penetration depth, once more ignoring factors such as load frame stiffness and thermally-induced relative position drift. IFMs currently do not have a displacement sensor dedicated to measuring the piezoelectric actuator's extension or contraction; nevertheless, IFMs are quantitative nanoindenters from the viewpoint of loading configuration. Solving the relevant torque balance equation yields the contact force. The rotational spring constant of the torsion bars does not enter into the calculation of contact force because the rotatable electrode is suppressed from rotating.
The IFM sensor is currently too large to be housed in a TEM holder; furthermore, the baseline control effort needed to maintain an extended-length indenter in the horizontal orientation will be highly dependent on TEM-holder rotation angle, as will be the maximum load available for nanoindentation. Nevertheless, actuatable capacitive transducers are highly attractive for quantitative in-situ TEM nanoindentation because their operation is not based on magnetic principles, they draw very little electrical current, thus they generate very little heat, and they possess favorable scaling laws for miniaturization.
The Detailed Description of the invention discloses a novel actuatable capacitive transducer in addition to other novel aspects of the invention. Yu et al. made an initial public disclosure on an alternative actuatable capacitive transducer in the on-line version of Reference 36 on Mar. 28, 2005. The Yu et al. alternative actuatable capacitive transducer clearly is not suitable for quantitative in-situ TEM nanoindentation as disclosed.
For these and other reasons there is a need for the present invention.